1. Field of the Invention
This invention relates generally to a system and method for determining whether a water vapor transfer (WVT) unit in a fuel cell system is operating properly and, more particularly, to a system and method for determining whether a WVT unit in a fuel cell system has cross-over leaks by comparing an output of a relative humidity (RH) sensor that measures the relative humidity in a cathode input line to a fuel cell stack and an RH value provided by a high frequency resistance (HFR) circuit that determines membrane humidity of the membranes within the fuel cell stack.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte there between. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically, but not always, include finely divided catalytic particles, usually a highly active catalyst such as platinum (Pt) that is typically supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow fields are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow fields are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The membrane within a fuel cell needs to have sufficient water content so that the ionic resistance across the membrane is low enough to effectively conduct protons. Membrane humidification may come from the stack water by-product or external humidification. The flow of reactants through the flow channels of the stack has a drying effect on the cell membranes, most noticeably at an inlet of the reactant flow. However, the accumulation of water droplets within the flow channels could prevent reactants from flowing therethrough, and may cause the cell to fail because of low reactant gas flow, thus affecting stack stability. The accumulation of water in the reactant gas flow channels, as well as within the gas diffusion layer (GDL), is particularly troublesome at low stack output loads.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will typically include significant water vapor and liquid water. It is known in the art to employ a water vapor transfer (WVT) unit to capture some of the water vapor in the cathode exhaust gas, and use the water vapor to humidify the cathode input airflow. Water in the cathode exhaust gas at one side of the water transfer elements within the WVT unit, such as membranes, is absorbed by the water transfer elements and transferred to the cathode air stream at the other side of the water transfer elements.
As discussed above, it is generally necessary to control stack humidity so that the membranes in the stack have the proper electrical conductivity, but where the flow channels do not become blocked by ice if the water freezes during system shut-down. It is known in the art to provide an RH sensor in the cathode air inlet of a fuel cell system to measure the humidification of the cathode inlet gas stream as it enters the stack. Using the measured inlet relative humidity and the water specie balance, or mass balance of water, the RH profile of the fuel cell system, including cathode air outlet flow, can be estimated. The ability of the RH sensor to provide an accurate reading of the RH is determined by the cost and complexity of the sensor. It is typical desirable to limit the cost of the sensor, which reduces its accuracy.
A technique for determining membrane humidification is known in the art as high frequency resistance (HFR) humidification measuring. HFR humidification measurements are generated by providing a high frequency component or signal on the electrical load of the stack so that a high frequency ripple is produced on the current output of the stack. The resistance of the high frequency component is then measured by a detector, which is a function of the level of humidification of the membranes in the stack. High frequency resistance is a well-known property of fuel cells, and is closely related to the ohmic resistance, or membrane protonic resistance, of the fuel cell membrane. Ohmic resistance is itself a function of the degree of fuel cell membrane humidification. Therefore, by measuring the HFR of the fuel cell membranes of a fuel cell stack within a specific band of excitation current frequencies, the degree of humidification of the fuel cell membrane may be determined. This HFR measurement allows for an independent measurement of the fuel cell membrane humidification, which may eliminate the need for RH sensors.
A typical WVT unit includes membranes made of a special material where the wet flow on one side of the membrane is transferred through the membrane to humidify the dry flow on the other side of the membrane. Because the material that makes up the membranes is relatively thin and the pressure on the cathode inlet side provided by the compressor is higher than the pressure at the cathode outlet side, WVT units sometimes fail where holes form in the membranes so that the airflow on the input side of the WVT unit flows directly to the output side of the WVT unit without passing through the fuel cell stack. Because there is a loss of airflow into the fuel cell stack, the oxygen that is available in the fuel cell stack to provide the reaction is reduced, which reduces the performance of the stack. Further, less airflow through the cathode flow channels as a result of the airflow cross-over reduces the amount of airflow that is able to remove water from the cathode flow channels, also affecting stack performance. Further, if the relative humidity of the cathode inlet air is different than what is detected, the cathode stoichiometry will be different than what is expected, which also affects stack performance.